Abstract
Twelve full-scale tests have been performed to investigate the underside problem and to determine the effectiveness of several specific guard designs in preventing underside type collisions. Conclusions based on the results of the crash tests and the computer simulations, are drawn concerning various aspects of the underside problem such as guard strength requirements, guard ground clearances, and the force-stroke requirements for yielding guard designs. Effects of differences in car weights and sizes, and truck-trailer structures, on the general underside guard problem are also considered.